This invention relates to improvements in a radar system for tracking a target at low elevation angles.
When a radar system attempts to detect targets such as aircraft entering at low altitudes, it is difficult to positively acquire the targets by the radar system unless the surface of the sea or earth above which the targets are moving is flat. This is because diffuse reflections occur on the rough surface.
In order to track targets such as aircraft at low elevation angles, the so-called array aperture sampling technique is known. According to this technique, a plurality, for example, N, antenna elements are arranged in a vertical array so as to transmit an electric wave toward a target at low elevation angles and receive an echo from the target. The echo includes a direct wave reflected from the target and a reflected wave or waves resulting from a mirror image and a diffuse reflection or reflections from the surface of the earth or sea. Thus the echo is subjected to the multipath effect.
An echo portion received by each of the antenna elements is resolved into a real and an imaginary part of a signal in the form of a complex number corresponding to that echo portion by a pair of phase detectors which are respectively supplied with sinusoidal signals in a quadrature relationship. By using those N complex signals, an electronic computer can calculate the directions of arrival of the echo such that minimum differences exist between the N complex signals and N complex outputs from the antenna elements obtained when imaginary directions of arrival of the echo whose number n is known are selected at will. Then, the direction of arrival of the direct wave has been determined by that calculated direction of arrival having a maximum elevation angle.
To determine an elevation angle of a target as described above has been disadvantageous in that an elevation tracking error is large when the surface of the earth or sea has a roughness which is large as compared with a wavelength transmitted by the antenna elements.
Accordingly, it is an object of the present invention to provide an improved radar system for tracking a target at low elevation angles with a high accuracy even in the presence of diffuse reflections from the rough surface of the earth or sea.